The present invention relates to seismic exploration and particularly to a method and apparatus for detecting and measuring up- and down-traveling seismic waves. In U.S. Pat. No. 2,740,945, there is disclosed a method for measuring up- and down-traveling seismic waves. In particular, the patent discloses the use of a pressure-sensitive seismic detector, such as hydrophone, in combination with a velocity-sensitive seismic detector, such as a geophone, to measure the seismic waves resulting from the seismic disturbances. The outputs of the two detectors can be either compared or combined to determine the direction of arrival of the seismic waves at the detectors. Also, the signals can be combined so as to substantially eliminate the downwardly-traveling waves which result from a reflected seismic wave being reflected a second time from an interface above the position of the detectors.
The patent describes the positioning of the combination velocity-sensitive and pressure-sensitive detectors in a fluid-filled borehole. The patent then describes that the signals from the detector can be transmitted to the surface where they can be recorded. The patent also describes the use of amplifiers and filters at the surface recording station to match the frequency and phase of the two signals.
While the above system describes the use of pressure-sensitive and velocity-sensitive detectors to detect up- and down-traveling waves, it only describes their use in a fluid-filled borehole. In fluid-filled boreholes, however, seismic waves trapped in the borehole, commonly called tube waves, have amplitudes which will exceed the normal amplitudes of seismic waves of interest. Thus, the tube waves will completely obscure the real signals of interest. Also, the use of filters in an attempt to match the phase and frequency of the received signals may eliminate certain frequencies of interest and consequently, not produce the desired results.